Nitriles, and particularly aliphatic nitrites, are obtained by the ammoxydation of olefins. Nitriles thus obtained inevitably contain a trace amount of an aldehyde as an impurity, which contaminates the system during the step of recovering the nitrile or in coloring the product nitrile. Furthermore, recent developments in chemical technology require higher purity nitrites. However, the volatility of a nitrile is often close to that of an aldehyde present therein as an impurity. Thus, the separation of these components by distillation is uneconomical, and requires many plates and a considerable amount of energy. Therefore, there is a need in the art to develop an efficient method for removing an aldehyde impurity from a nitrile.
Several proposals have been made to date including, for example, a method of adding sodium hydrogennitrite to methacrylonitrile to remove methacrolein in the form of an addition salt (see JP-A-57-62247, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), a method of adding acetylacetone, etc. to acrylonitrile and removing the reaction product between the acetylacetone and acrolein by distillation or like means (see JP-B-57-26586, the term "JP-B" as used herein means an "examined published Japanese patent application"), and a method of using ozone for removing acrolein from acetonitrile (see DD 217212).
Methods of using an ion exchange resin for removing acrolein from acrylonitrile have also been proposed. For example, the use of a porous anion exchange resin having a primary amino group and/or a secondary amino group as an ion exchange group (see JP-B-58-1108) or an anion exchange resin gel (see JP-A-58-134063) is known. These methods show promise for specifically removing aldehydes without producing other impurities in the system.
However, according to the method of using an anion exchange resin having a primary amino group and/or a secondary amino group as an ion exchange group, the amino group and the aldehyde group undergo an amino-carbonyl reaction to form a covalent bond. Thus, the aldehyde group cannot be released in a usual manner. As a result, the spent ion exchange resin is hardly regenerated and must be discarded, thus adding the cost of disposal to this process.